The intentional drilling of directional boreholes began in South Africa nearly eighty years ago. A description of this process, by John Hoffman, is found in Bulletin 91 of the American Institute of Mining Engineers (Apr. 11, 1912). The method entailed sequentially drilling two straight but intersecting boreholes.
The technique of intentionally deviating the borehole of an oil well was first widely used in the 1920's or 1930's in the Huntington Beach field. Prior to invention of that technique, offshore wells were drilled from piers placed some distance from the shoreline. The number of piers began to impede ship traffic in some Pacific harbors and eventually led to the outlawing of such piers along some portions of the North American West Coast.
One driller's solution to the problem of outlawed piers entailed placing a rig on the shore and deviating the borehole to reach a producing formation offshore. Although the technique worked well, the driller's failure first to gain permission of either the state or the harbor authorities caused the technique to be disfavored.
However, the legitimate use of controlled directional drilling in shoreline drilling is not the only instance in which the process is useful.
Controlled directional drilling can be used to drill a borehole anywhere a surface obstruction prevents placement of the well site over the point where a well is to be bottomed out. The obstruction could be a hill, marsh, swamp, river, or freeway.
A deviated well may also be used to control another well which is burning or blowing formation fluids out of control. The deviated well is drilled to intersect a region near the borehole of the offending well. High pressure mud is pumped through the deviated borehole into the other borehole to control the formation fluids being lost to the fire or blowout.
Controlled deviated drilling techniques may be used in the optimization of reservoir pressure. For instance, if an initial well bottoms out in the upper end or gas cap of a producing formation, it may be wise to plug a lower portion of the well and deviate the borehole from the plugged point into a lower portion of the formation to recover liquid petroleum. Gas pressure in the cap is often the major driving force behind the petroleum liquids produced from the same formation. Production of the gas would lower its driving force on the liquid petroleum and ultimately lower the overall recovery from the formation.
Probably the most common instance of the use of controlled directionl drilling is found in the ubiquitous offshore platform. It is common to drill dozens of wells from a single platform. The expense of building an offshore platform for each well should be apparent to even the casual observer. In any event, the borehole for each well drilled from a platform typically follows a near vertical path to a specified depth into the sea bed and then quickly veers away from its neighboring wells.
Controlled directional drilling is not the only reason for the existence of deviated wellbores. The process of drilling obliquely from a soft geological layer into a relatively harder subterranean strata will cause the drill bit to serve from a vertical path. Similarly, insufficient drill collar weight on the lower end of the drill string will cause the bit to wander from the vertical during drilling. Neither situation is a desirable one and much care is taken by the driller to avoid their occurrence.
But whatever the reason for the existence of a deviated wellbore, the step of logging the wellbore once it has been drilled presents special challenges.
Logging a well is done to obtain a wide diversity of information using equally diverse types of instrumentation. In the normal course of events, the well is logged after drilling. Many wells require a number of logging runs to evaluate various wellbore intervals. The law of some locales, e.g., Norway, require that the entire length of the wellbore be logged. Drilling rigs are often rented on a daily basis and consequently anything subtracting from time available to drill is to be avoided. It occasionally may be necessary to log the well before the planned total depth ("TD") is attained to make sure that, e.g., a desired formation is penetrated. The surrounding wellbore formations are scanned to provide information concerning porosity, density, lithology, and characteristics of the formation fluids. Physical parameters of the borehole, such as its diameter, are measured so that subsequent casing and cementing steps may be efficiently completed.
There are two methods typically used for the physical step of placing a logging tool in the wellbore and then withdrawing it. The first method is practiced with the drill string out of the hole. A downhole tool or sonde, often weighin g several hundred pounds, is lowered in the open borehole upon a logging cable hanging from a pulley on the surface. If the borehole is vertical or nearly so, then gravity may take the logging sonde to the bottom of the hole. However, if the borehole has a dogleg or is otherwise deviated, reliance on gravity to carry the sonde to bottom is a questionable proposition. Even if the borehole is deviated, the overall logging costs may be minimized by first attempting this method of inserting the sonde and, if unsuccessful, proceeding to another method.
An improvement to the basic gravity impelled sonde is found in U.S. Pat. No. 4,031,750, Toyoumans el al. This logging instrument utilizes a linear electric motor attached to a set of vanes extending out from the instrument body. The sonde is dropped in the open borehole until it reaches a point where it no longer moves down. The electric motor is then actuated and the vanes reciprocate on the outside of the body and "rows" the device down the borehole. This apparatus apparently is not in wide use.
One logging method used after failure of the free-fall sonde method uses a drill string completely made up of drill pipe, i.e., having no drill collars or drill bit at its lower end. The open-ended drill string is generally run into the hole to a point below the region to be logged. A special logging sonde, having a very narrow diameter, is then attached to a logging cable and pumped down the drill string and out into the open hole below the drill string's lower end. The logging cable is connected to a recording instrument at the surface. The drill string is raised about 90 feet. The well in then logged by pulling the sonde up and recording the data it gathers. Once the sonde reaches the lower end of the drill pipe, data is no longer recorded and the sonde is pulled all the way to the surface through the drill string. A stand or treble (approximately 90') of pipe is remove from the string. The sonde is again inserted into the drill string and pumped out its lower end. This places the sonde at the point at which logging was terminated in the prior pass. The drill string is again pulled up about the length of a stand of pipe and the sonde subsequently follows it up logging the then-vacated ninety feet. The sonde, once again, is pulled to the surface and another stand of drill pipe removed from the string. This process is repeated ninety feet at a time until a sufficient amount of the wellbore is logged. Obviously this process is slowly and laborious. Only ninety feet of the well is logged with each pass.
A variation of this process, used when the wellbore has a nonvertical section which prevents insertion of a sonde to a desired interval but has a lower vertical leg, entails running the drill pipe down through the wellbore deviation and into the vertical region above the interval to be logged. The sonde is pumped out through the drill pipe and allowed to fall by gravity through the interval to be logged. Logging can then be carried out using the operation described above.
One method for increasing the length of wellbore logged with each pass of the sonde is found in U.S. Pat. No. 4,062,551, to Base. This process uses a short sub placed in drill string which allows the logging cable to pass through the drill string wall at some midpoint within the well. The well may be logged for a distance equal to the length of pipe between the pass-through sub and the surface before the sonde is pulled from the drill string. Some portion of the remaining drill string must then be pulled to re-install the pass-through sub in the drill string.
Similar suggestions for pass-through subs are found in U.S. Pat. No. 4,200,297 to Tricon, issued Apr. 29, 1980, and U.K. Patent Application GB No. 2,094,865A, to Institut Francais de Petrole ("IFP"), published Sept. 22, 1982. Both suggest using a drill string as the impetus for getting a sonde to the bottom of a borehole for logging purposes. Tricon, however, uses the drill stem to drive a drilling head; the drill stem does not rotate. IFP, on the other hand, uses a drill string as an upper part of a suite of logging tools to place the logging tools in a highly deviated portion of a borehole.
Another logging suite placed at the end of a series of drill pipe sections is disclosed in U.S. patent application No. 528,349 to Davis and Knight, filed Aug. 31, 1983.
Each of these devices required some manner of making and breaking electrical connection between the surface and the instrument package. IFP suggests one such connector having a female portion or socket which is pumped down on the end of a cable and further having a male portion or plug which is fixedly mounted above the tool string. Although IFP suggests that is device is especially suitable for use in deviated wells, at least one problem in operation would appear to occur when downhole debris is found in the path of the female portion. Drilled rock chips often gather on the deviated drill stem wall and may enter the pumped down socket. Similarly, debris would be expected to collect at the base of the plug portion. The electrical connection may be hard to make using the IFP connector under adverse circumstances.
Other connectors which are viewed as having similar problems are shown in U.S. Pat. No. 2,250,463 to Boynton, issued July 29, 1941; U.S. Pat. No. 3,976,347 to Cooke et al, issued Aug. 24, 1976; and U.S. Pat. No. 4,130,169 to Denison, issued Dec. 19, 1978.